1. Field of the Invention
This invention relates to bubble jet ink printing systems and more particularly to a bubble jet ink priting device having an improved bubble generating means.
2. Description of the Prior Art
Generally speaking, ink jet printing systems can be divided into two types; viz, continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, so that the stream breaks up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is to not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. The major components of one type of drop-on demand system are an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the systems ability to produce high quality copies. The drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The bubble jet concept is the other drop-on-demand system, and it is very powerful because it produces high velocity droplets and allows very close spacing of nozzles. The major components of the second type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle.
As the name suggests, printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. The process is ready to start all over again as soon as hydrodynamic motion of the ink stops. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous steam counterparts and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system starts with a current pulse through the resistive layer in the ink filled channel, the resistive layer being near the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated (far above its normal boiling point) and for water based ink, finally reaches the critical temperature for bubble nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. The bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor which, of course, removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle. Once the excess heat is removed, the bubble collapses on the resistor. The resistor at this point is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble which tends to erode it. The ink channel then refills by capillary action. The entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened.
Investigation and experimentation have revealed that the bubble jet concept of the prior art encounters a critical problem of resistive layer lifetime because of inefficient thermal heat transfer to the ink, requiring higher temperatures in the resistive layer, as well as because of the cavitational forces during the bubble collapse. The lifetime of the resistive layer, of course, determines the useful life of the bubble jet ink printing device. The present invention overcomes this resistive layer wear and shortened operating lifetime by providing a more efficient, lower power consumming bubble generating means as will be more fully discussed below.
One of the most widely used prior art structures for a bubble generating means 50 for a typical bubble jet ink printing device, such as like one depicted in FIG. 1, is shown in FIG. 5. It is a layered, resistive thin film device having a support structure 51 which must have a high thermal conductivity. The support structure is generally silicon or a ceramic material such as aluminum oxide (Al.sub.2 O.sub.3). An underglaze layer 52 of sputtered silicon dioxide (SiO.sub.2) is placed on the support structure having a thickness of 2 to 5 microns. A resistive material such as zirconium boride (ZrB.sub.2), is sputtered on the underglaze to form resistor 53. The thick SiO.sub.2 underglaze is necessary to allow some thermal isolation between the thermally conductive substrate and the resistor. The underglaze has poor thermal conductivity compared to the substrate. An unattractive feature of the underglaze is that the contact between the resistor and electrical leads also gets hot because the contact area is thermally isolated as well. The resistor is connected to external drive electronics (not shown) by aluminum leads 54. A 0.5 micron sputtered SiO.sub.2 film 55 is used to dielectrically isolate the resistor 53 and the aluminum leads 54 from the conductive ink which is contained in the channel 56 of channel plate 57 shown in dashed lines. A one micron tantalum (Ta) layer 58 is sputter deposited on the resistor. The purpose of the Ta layer 58 is for the protection of the SiO.sub.2 film from damage from the bubble collapses. The SiO.sub.2 film is attacked quite readily by heat and cavitational forces generated by the collapsing bubble.
The structure of the bubble generating means 50 is considered adequate but quite expensive to manufacture and is inefficient in operation. The SiO.sub.2 film 55 is too thick to allow efficient heat transfer from the resistor 53 to the Ta layer 58. The thickness of the SiO.sub.2 film is mandated by the need for good dielectric isolation. It is well known in the industry that production of thin sputtered SiO.sub.2 films having thicknesses of less than 5000 angstrom (.ANG.) with good integrity is not easily achieved at high yield, especially since the SiO.sub.2 film must cover a step at the edge of the resistor. To bring the yield up to commercially acceptable percentages, the thickness must be increased. Another important shortcoming of the prior art design is that active drive devices cannot be easily integrated on the support structure 51 without the addition of many process steps that necessitate enlarging the size of the printing head containing the array of bubble jets. Increased process steps increase cost, while compact printing heads, especially those for carriage printers, is highly desirable.
U.S. Pat. No. 4,251,824 to Hara et al discloses a bubble jet drop-on-demand system. FIG. 7A and 7B therein show a single resistive layer for each nozzle. Thermal energy is applied to the ink by the resistive layer to bring about the change in state of the ink to develop bubbles and discharge droplets from the nozzle for recording.
U.S. Pat. No. 4,410,899 to Haruta et al discloses a method of expelling a droplet by producing and eliminating a bubble in the ink passageway in such a way that the maximum bubble volume does not block the ink flow in the passageway.
U.S. Pat. No. 4,412,224 to Sugitani discloses a method for forming the ink channels in the bubble jet printing head by a "photo-forming" technique directly on the substrate having the circuitry and resistive layers.
The prior art bubble jet devices provide close spacing of ink channels and generation of high velocity droplets so that high speed and high resolution printing is possible. The disadvantages of prior art devices are that they require an expensive manufacturing technique and that they provide an inefficient use of the thermal energy. If the bubble jet could be made more thermally efficient, then inexpensive MOS type circuitry (N-MOS) can be used to drive the head instead of the more expensive bipolar circuitry. It is, of course, desirable and cost effective to have a resistor structure which is immediately and simply integrated and the same wafer with MOS drive electronics, preferably without additional process steps. As will be seen below, these improvements form the basis of the present invention.